圖1. ORF2p逆轉錄酶的活性位點
The active site of the ORF2p reverse transcriptase
Billions of years ago, as primitive lifeforms were becoming more complex, a selfish genetic component became a sort of genome colonizer. Using a copy-and-paste mechanism, this pernicious bit of code replicated and inserted itself again and again into a variety of genomes.
數十億年前，隨著原始生命形式變得越來越複雜，一種自私的遺傳成分，變成了一種基因體殖民者。使用複製貼上機制，此有害的遺傳密碼，一再複製並自行嵌入到各種基因體中。
Over time, all eukaryotic organisms inherited the code—including us. In fact, this ancient genetic element wrote about one-third of the human genome—and was considered junk DNA until relatively recently.
隨時間推移，所有真核生物繼承了此遺傳密碼，包括咱們。事實上，此遠古遺傳成分，顯露了大約三分之一的人類基因體，直到最近才被認為是垃圾DNA。
This genetic component is known as LINE-1, and its aggressive intrusion into the genome can wreak havoc, leading to disease-causing mutations. A key protein called ORF2p enables its success—meaning understanding ORF2p’s structure and mechanics could illuminate new potential therapeutic targets for a variety of diseases.
此遺傳成分被稱為LINE-1，而其侵犯性的侵入基因體中，會造成嚴重破壞，導致致病突變。一種被稱為ORF2p的關鍵蛋白質，使其得以奏效。這意味著，瞭解ORF2p的結構及機制，可能為多種疾病，闡釋新的潛在治療標的。
Now, in collaboration with more than a dozen academic and industry groups, Rockefeller scientists have rendered the protein’s core structure in high-resolution for the first time, revealing a host of new insights about LINE-1’s key disease-causing mechanisms. The results were published in Nature.
目前，與十多個學術及產業團體合作。美國洛克斐勒大學科學家們已經首度，以高解析度呈現該蛋白質的核心結構，揭露了諸多有關LINE-1關鍵致病機制的新洞察力。此些研究結果，發表於《自然》期刊。
“The work will facilitate rational drug design targeting LINE-1 and may lead to novel therapies and strategies to combat cancer, autoimmune disease, neurodegeneration, and other diseases of aging,” says senior author John LaCava, a research associate professor at The Rockefeller University.
資深撰文人，洛克斐勒大學研究副教授，John LaCava宣稱：「這項研究能促進，鎖定LINE-1之合乎的藥物設計，且可能導致對抗癌症、自體免疫疾病、神經退化性疾病與其他老化疾病的新療法及策略。」
LINE-1 is a retrotransposon, a kind of mobile genetic code that translates RNA back into DNA while replicating and writing itself into different places throughout an organism’s genome. There are different kinds of retrotransposons, including endogenous retroviruses (ERVs), which resemble HIV and Hepatitis B (HBV).
LINE-1是一種逆轉錄轉座子。也就是，當複製並自身寫入整個生物基因體不同位置時，將RNA轉譯回DNA的一種移動性遺傳密碼。有多種不同類型的逆轉錄轉座子，包括類似人類免疫缺乏病毒(HIV：Human Immunodeficiency Virus)及B型肝炎(HBV)的內源性逆轉錄病毒(ERVs)。
LINE-1’s origin is unclear, but it has an evolutionary connection to group II introns, a class of ancient mobile elements dating back about 2.5 billion years. Retrotransposons like LINE-1 have been evolving with their host organisms for 1 to 2 billion years.
LINE-1的起源不詳。不過它具有與一類，追溯到大約25億年前之遠古移動性分子的第二群內含子，演化上的關聯性。像LINE-1的逆轉錄轉座子，一直與其宿主生物一起演化，達10到20億年。
“It’s an ongoing battle between LINE-1 trying to insert itself and the host protecting its own genome,” says co-first author Trevor van Eeuwen, a postdoctoral fellow in Rockefeller’s Laboratory of Cellular and Structural Biology.
首要合撰人，洛克斐勒大學細胞暨結構生物學實驗室博士後特別研究員，Trevor van Eeuwen宣稱：「這是試圖自行嵌入的LINE-1與保護自身基因體之宿主間，進行中的一場搏鬥。」
Millions of genetic fragments derived from LINE-1 are found in our cells. The vast majority are inactive evolutionary relics, evidence of failed attempts to hijack the replication machinery. But about 100 LINE-1s are operational—and usually not helpful. One protein produced by LINE-1, known as ORF1p, is churned out by cancer cells, as a recent study by LaCava, Michael P. Rout, and their collaborators described.
數百萬源自LINE-1的基因片段，在我們的細胞中被發現。絕大多數是不活躍的演化遺物，是試圖劫持複製運作部分，失敗的證據。不過，大約100個LINE-1處於正常運作狀態。但是，通常非有益。如同LaCava、Michael P. Rout及其共同研究者們，最近一項研究所描述的，一種由LINE-1所產生，被通稱為ORF1p的蛋白質，是由癌細胞大量產生的。
LaCava and Martin Taylor, of Massachusetts General Hospital and Harvard Medical School, have collaboratively studied LINE-1 and its proteins for more than a decade, but because ORF2p expresses so lowly and infrequently, it has remained poorly understood.
隸屬美國馬薩諸塞州總醫院及哈佛醫學院的LaCava及Martin Taylor，已經共同研究LINE-1及其蛋白質，達十多年。不過，由於ORF2p作出表現的程度極低且不頻繁。因此，它仍然一直不被充分瞭解。
“LINE-1 has been so difficult to study because it has very weird features,” LaCava says. “For instance, it has an unusual replication cycle and the ORF2p protein that no one’s been able to capture. But Marty and I eventually reached a place where our research on it was mature enough so that we could begin to study its structure.”
LaCava宣稱：「LINE-1一直很難研究，因為它有非常怪異的特性。譬如，它具有不尋常的複製週期，及無人能夠捕獲的ORF2p蛋白質。不過，Marty與我最終達到了，我們對其研究夠成熟之處，以致於我們能開始研究它的結構。」
Taylor made key advancements in purifying the full-length ORF2p as well as a shorter “core” version that facilitates L1 replication; these advances facilitated the breakthroughs that followed.
除了促進L1複製的較短“核心”部分之外，在純化全長的ORF2p方面，Taylor也獲得了關鍵性進展；此些進展促進了隨後的諸多突破。
Using a combination of x-ray crystallography and cryo-EM, the research team discovered two novel folded domains within ORF2p’s core that contribute to LINE-1’s ability to make copies of itself.
藉由結合X-射線晶體學及低溫電子顯微鏡學(cryo-EM：Cryogenic electron microscopy)，該研究團隊，在有助於LINE-1自行複製之能力的ORF2p核心內，發現了兩個新的折疊結構域。
ORF2p has structural adaptations uniquely suited for these endeavors, says van Eeuwen. It’s a sort of jack-of-all-trades protein, capable of handling everything from replication to insertion. But while most viruses need potentially hundreds of reverse transcriptase proteins to replicate, ORF2p does it all.
van Eeuwen表示，ORF2p具有諸多獨特適合此些嘗試的結構適應性。它是一種能夠處理，從複製到嵌入之所有事情的萬事通蛋白質。不過，儘管大多數病毒，潛在上需要數百個逆轉錄酶蛋白質來進行複製，ORF2p完全做到。
Yet when LINE-1 is activated in the cytoplasm, “it acts like a viral mimic. It creates RNA:DNA hybrids that look like a viral infection when they’re sensed,” van Eeuwen notes notes.
然而，於細胞質中，當LINE-1被激活時。van Eeuwen 特別提及：「它起如同病毒模仿者的作用。創造了RNA:DNA混合體，當它們被檢測到時，看起來就像一種病毒感染。」
This viral mimicry suggests a possible solution to the puzzle of how ORF2p activates the innate immune system, contributing to autoimmune disease and other conditions. Their research found that interactions with genetic material in the cytoplasm activate the cGAS/STING antiviral pathway.
這種病毒模仿暗示了，ORF2p如何激活先天免疫系統、導致自體免疫疾病及其他疾病之謎團，一種可能的解答。他們的研究發現了，與細胞質中遺傳物質的交互作用，激活了cGAS/STING(先天免疫系統的一個組成部分)的抗病毒途徑。
In turn, that pathway causes cells to produce interferons, stimulating the immune system and leading to inflammation, in a manner analogous to what happens during an infection by a virus.
依序，那途徑以一種類似病毒感染期間發生的方式，導致細胞產生刺激免疫系統及導致炎症的干擾素。
“Its main function appears to be to proliferate copies of itself, and as LINE-1 moves sequences around, there’s a chance those sequences could break a gene,” he says. “But there’s also a chance they could create new genetic elements or novel functionalities that are beneficial to the host.”
他宣稱：「其主要功能似乎是增殖自身的複製。因此，當LINE-1到處移動序列時，那些序列有可能破壞基因的機會。不過也有，它們可能創造對宿主有益之新遺傳成分或新功能的機會。」
In the future, the researchers will seek to solve the two newly discovered core domains and understand their functions. In the meantime, “our structural elucidation of ORF2p lays the groundwork for future studies needed to dissect and improve our understanding of the LINE-1 insertion mechanism, its evolution, and its roles in disease,” van Eeuwen says.
於未來，此些研究人員將尋求解決，這兩新近發現的核心領域，並瞭解其功能。在此同時，van Eeuwen宣稱：「我們有關ORF2p結構的闡釋，為未來有必要的諸多研究奠定了基礎，以剖析及提高我們有關LINE-1嵌入機制、其演化及其在疾病方面之角色的理解。」
They also want to explore the potential clinical applications of their findings. Because there is a kinship between retrotransposons and retroviruses, in the current study they tested treatments for the retroviruses HIV and HBV to see if they would inhibit LINE-1. They did not, suggesting that the design of therapeutics will have to be tailored to LINE-1’s unique characteristics.
他們也想探索，其此些發現的潛在臨床應用。因為，在逆轉錄轉座子與逆轉錄病毒間，有種相似性。在當前的研究中，他們測試了，HIV及HBV逆轉錄病毒的治療方法，以瞭解是否它們能抑制LINE-1。它們沒有抑制，這暗示療法的設計，將必須針對LINE-1的獨特特性，量身訂做。
“The work opens the door to rational drug design of better LINE-1 inhibitors, and we hope these will lead to clinical trials soon,” LaCava says.
And, as Rout adds, “this study also underscores the potential of integrating multiple kinds of data—and multiple labs’ expertise—to solve fundamental biomedical questions.”
LaCava宣稱：「該項研究開啟了，更佳LINE-1抑制劑，合乎邏輯之藥物設計的途徑。因此，我們希望此些很快會導致臨床試驗。」而且，正如Rout 補充的，「這項研究也強調了，整合多種數據及多個實驗室的專業知識，來解決基本生物醫學問題的潛力。」
網址：https://www.rockefeller.edu/news/35133-revealing-how-an-ancient-genetic-invader-inhabits-our-dna/
翻譯：許東榮
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